A. Field of the Invention
The invention relates to a spectral sensor for checking documents of value and a method for checking documents of value with the help of the spectral sensor.
B. Related Art
For checking documents of value there are usually employed sensors with which the type of the documents of value is determined and/or with which the documents of value are checked for authenticity and/or their state. Such sensors are employed for checking documents of value such as e.g. bank notes, checks, identity documents, credit cards, check cards, tickets, vouchers and the like. The check of the documents of value is effected in an apparatus for processing documents of value in which one or several different sensors are contained, depending on the properties of the documents of value to be checked. Upon checking, the documents of value are usually scanned by the sensor, whereby the sensor and the document of value are moved relative to each other.
With a multiplicity of sensors, the documents of value are illuminated with light sources of certain colors, in order to ascertain from the remission of the documents of value for these colors the visually visible color of the document of value. Corresponding to the three different color receptors of the human eye, these sensors have only three color channels which are realized e.g. by red, green and blue light-emitting diodes (RGB sensors). With such optical sensors which have only three color channels, however, no spectral intensity distribution of the light emanating from the document of value can be recorded.
For recording a spectral intensity distribution, there are known spectral sensors which illuminate the documents of value with white light and detect in spectrally resolved fashion the light remitted by the documents of value. With such spectral sensors, there is employed a diffraction grating for the spectral splitting of the light remitted by the documents of value. The spectral splitting, however, requires a relatively long ray path from the diffraction grating to the detector line, so that such spectral sensors need a large installation space. Furthermore, the spectral range which can be captured with such a spectral sensor is relatively narrow, so that no spectral intensity distribution over a wide spectral range can be recorded therewith. Because the diffraction gratings are optimized for a certain wavelength, so that the reflection coefficient of the grating for the light of this wavelength is as great as possible. For wavelengths deviating therefrom, however, there occurs a strong drop in the reflection coefficient of the diffraction grating, so that of the light of these wavelengths only very low light intensities are available for the detection.